This invention relates to a phase control circuit and a magnetic reproducing apparatus using the same, and more particularly to such a circuit which contributes to an improved transition stability of a rotating apparatus with speed and phase control, and to such an apparatus wherein a transverse swing of the reproduced picture due to an intermittent transporting is avoided.
Phase control circuits to control the rotational speed of a rotating apparatus so as to be constant and also to control the rotational phase thereof to maintain a certain phase difference with respect to a reference signal are widely used. Now, let us consider the stability of a system after the rotational speed of a rotating apparatus (to be herein mentioned with respect to a motor) encounters an abrupt and predictable change, either intentionally or incidentally, under the condition that the rotational speed and phase are controlled by sampling and the sampling frequency of the speed control is higher than that of the phase control.
A timing chart for prior art systems is shown in FIG. 1. Suppose a speed change command is given during time t.sub.1 and t.sub.2 as in FIG. 1(A), the speed control system responds quick enough as in FIG. 1(B). But as to the phase control system, the response is relatively slow due to the slow sampling rate as shown in FIG. 1(D), so that phase error lasts after speed has settled down, and the response speed depends upon the gain or response of the phase control system. Hence, the phase deviation .theta. as shown in FIG. 1(C) always occurs and the phase change after the time t.sub.2 is inevitable due to the control system used to reduce the phase error. This is a significant disadvantage in such a system in that the transition phase change is more important than the stable phase shift. As an example of such a system, there is known a magnetic reproducing apparatus which intermittently transports a magnetic media to perform a slow motion reproduction. A video signal magnetic reproducing apparatus (VTR) using a magnetic tape (tape) is mentioned herein.
FIG. 14 shows a construction of a tape transport control system and the a rotatary head control system. The tape 31 is transported by a pinch roller 32 and a capstan 34 which is driven by a capstan motor 33. The capstan motor 33 is controlled by a control circuit 37 which accepts an output from a control head 35 and an intermittent travel control signal 36 to obtain a certain intermittent transport. The rotary heads 38 are driven by a cylinder motor 39.
Speed control circuit 44 controls the speed and phase of the cylinder motor 39 with an output of a speed detector 43 used to detect the rotational speed thereof and an output of a phase comparator circuit 42 used to compare the phase deference between an output of a phase detector used to detect the rotational phase of the cylinder motor 39 and a phase reference signal (III) which is an output of a reference signal generator 41. (I) and (II) show the direction of the tape 31's travel and rotary head 38's rotation, respectively.
FIG. 15 shows the operate timing of FIG. 14. The tape 31 is transported with the speed change as in FIG. 15(B) during the time t.sub.1 and t.sub.2 by the intermittent travel control signal 36 (FIG. 15(A)). In this case, the relative speed of the rotary heads 38 and the tape 31 become temporarily slower as shown in FIG. 15(C). Hence, the time axis of the reproduced video signal fluctuates. More particularly, fluctuation of a horizontal synchronizing signal period in the video signal cause a transverse swing of the reproduced picture in relation the the response characteristics of a synchronizing circuit within a television receiver. In order to compensate for this time axis fluctuation, the relative speed of the tape 31 and the rotary heads 38 is kept constant at every moment by modifying the rotational speed of the cylinder motor 39 to be accelerated as shown in FIG. 15(D). The cylinder motor 39, however, is supplied with the phase control, as mentioned above, to modify the rotational phase with respect to, the phase reference signal III as shown in FIG. 15(E). That is, the rotational phase advances in accordance with the rotational speed change, and the operation to correct the phase error is performed by the phase control circuit after the speed change has settled down because the sampling of the phase control circuit is carried out at the timing as shown in FIG. 15(F).
The aforementioned problem is caused by the relationship of the response speed of the speed and phase control systems and the period of the tape 31's intermittent travel, the problem being inevitable with the prior control system as in FIG. 14. Consequently, even after the speed change of the cylinder motor 39 has settled down, the time axis fluctuation lasts too long to exhibit the transverse swing of the reproduced picture, which is a great disadvantage.